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Shareholder Report
 6 Months Ended
May 31, 2025
USD ($)
shares
Shareholder Report [Line Items]  
Document Type N-CSRS
Amendment Flag false
Registrant Name Fidelity Advisor Series IV
Entity Central Index Key 0000719451
Entity Investment Company Type N-1A
Document Period End Date May 31, 2025
Fidelity Limited Term Government Fund  
Shareholder Report [Line Items]  
Fund Name Fidelity® Limited Term Government Fund
Class Name Fidelity® Limited Term Government Fund
Trading Symbol FFXSX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity® Limited Term Government Fund for the period December 1, 2024 to May 31, 2025.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at fundresearch.fidelity.com/prospectus/sec. You can also request this information by contacting us at 1-800-544-8544 or by sending an e-mail to fidfunddocuments@fidelity.com.
Additional Information Phone Number 1-800-544-8544
Additional Information Email fidfunddocuments@fidelity.com
Additional Information Website fundresearch.fidelity.com/prospectus/sec
Expenses [Text Block] What were your Fund costs for the last six months?
(based on hypothetical $10,000 investment)
 
 
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
Fidelity® Limited Term Government Fund 
$ 15 
0.30%
  
Expenses Paid, Amount $ 15
Expense Ratio, Percent 0.30%
Net Assets $ 174,818,479
Holdings Count | shares 235
Investment Company Portfolio Turnover 47.00%
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of May 31, 2025)
 
KEY FACTS 
  
  
Fund Size
$174,818,479
  
Number of Holdings
235
  
Portfolio Turnover
47%
  
Holdings [Text Block]   COUPON DISTRIBUTION (% of Fund's net assets) 0.01 - 0.99% 2.6   1 - 1.99% 5.3   2 - 2.99% 18.5   3 - 3.99% 28.2   4 - 4.99% 36.7   5 - 5.99% 1.9   6 - 6.99% 2.0   7 - 7.99% 0.0         U.S. Treasury Obligations 74.8 CMOs and Other Mortgage Related Securities 17.2 U.S. Government Agency - Mortgage Securities 3.2 Options 0.2 Short-Term Investments and Net Other Assets (Liabilities) 4.6 ASSET ALLOCATION (% of Fund's net assets)     U.S. Treasury Obligations - 74.8           CMOs and Other Mortgage Related Securities - 17.2   U.S. Government Agency - Mortgage Securities - 3.2   Options - 0.2                              Short-Term Investments and Net Other Assets (Liabilities) - 4.6  
Largest Holdings [Text Block] TOP HOLDINGS (% of Fund's net assets)     US Treasury Notes 74.1   Freddie Mac Multifamily Structured pass-thru certificates 14.6   Fannie Mae Mortgage pass-thru certificates 1.9   Uniform Mortgage Backed Securities 1.1   Fannie Mae Guaranteed REMIC 1.1   US Treasury Bonds 0.7   Ginnie Mae Mortgage pass-thru certificates 0.7   Freddie Mac Gold Pool 0.6   Ginnie Mae REMIC pass-thru certificates 0.1   Fannie Mae 0.1     95.0